According to a study released Wednesday by STELLAService, customers hoping to get a customer service question answered via Twitter can really only rely on two companies to get back to them every time: Zappos and L.L. Bean.

In the survey of the top 25 online retailers, only Zappos and L.L. Bean proved 100 percent reliable.

For the study, STELLAService's mystery shoppers tweeted basic customer service questions to the official Twitter accounts of the Internet's top 25 retailers every day for 45 days beginning April 1. The company than tracked the frequency and speed of each company's replies.

Of all those companies, only two—Zappos and L.L. Bean—replied to every single tweet within 24 hours. Rounding out the top five most reliable Twitter performers were Overstock.com (replying to 98 percent of all tweets within 24 hours), Dell (98 percent), and Best Buy (89 percent).

All told, however, among the top 25 retailers, an average of only 44 percent of customer tweets were answered at all.

STELLAService CEO Jordy Leiser says the study reveals that most retailers are paying short shrift to quality customers who are both willing to spend more than the average shopper and who place enormous importance on quality customer service. Citing a recent American Express Global Customer Service Barometer, Leiser said people who have used social media for customer service at least once in the past year are willing to spend 21 percent more with companies that provide great service.

"It's becoming clear that social media-savvy consumers are an incredibly important segment of any company's customer base, both because they have higher service expectations and they have wide broadcast networks for sharing their experiences with others," Leiser wrote in the report. "By failing to take Twitter seriously as a customer service channel, companies may be inadvertently ignoring some of their best customers at their own peril."

Given the real-time nature of Twitter and users' craving for immediacy, the study also revealed average response times for the five most reliable retailers. Zappos proved the speediest, posting an average response time of 54 minutes and 37 seconds, the only retailer to clock in at less than an hour, followed by Best Buy (1:47:20), Overstock (1:53:33), Dell (2:28:31), and L.L. Bean (3:55:07).

STELLAService is an independent company that measures and rates the customer service performance of online retailers. Leveraging a nationwide network of full-time mystery shoppers, STELLAService evaluates thousands of retailers each year using more than 350 metrics, from the quality of live customer support to the speed of refunds.